The present invention relates to apparatus and method for motor overload protection and, more particularly, to apparatus and method for motor overload protection which uses an elapsed time signal for enabling computation of motor temperature independently of any temporary power interruption.
Motor overload protection devices are employed for avoiding damage to electric motors during an overload condition. For instance, if the overload condition is left uncorrected, any current which substantially exceeds the ampere-rating of the motor can result in motor temperatures being sufficiently high for damaging or degrading winding insulation. This degradation can eventually cause electrical shorts in the windings of the motor and this, in turn, can ultimately lead to a catastrophic failure. However, motor overload protection devices proposed heretofore suffer from various significant disadvantages. For example, devices have been proposed which are controlled by a microprocessor. Although the microprocessor provides a greater flexibility in the control features of the overload protection device by modeling the thermal characteristics of the motor, such microprocessors must generally remain electrically powered, even when power is disconnected from the motor, in order to calculate the temperature of the motor. Thus, this type of continuously powered devices are vulnerable to any temporary power interruption since, as long as the microprocessor is unpowered, the microprocessor is simply unable to track any information indicative of motor cooling. This is particularly undesirable since the lack of such information essentially disables or prevents the microprocessor from accurately resuming computation of motor temperature once power is restored. Although schemes which employ battery backup and/or power backup capacitors may allow for the microprocessor to track or compute the thermal state of the motor based on its thermal characteristics when the main power source is interrupted, it will be appreciated that these schemes generally result in devices that are relatively expensive, bulky, complicated and generally require more software code due to their inherent complexity.
U.S. Pat. No. 5,303,160 proposes a scheme that somewhat improves the susceptibility of microprocessor-based devices to temporary power interruptions by using an external circuit, such as a resistance capacitance (RC) network having a generally fixed time constant, for storing an analog signal that mimics or is equivalent to the thermal characteristics, such as the I.sup.2.sub.1 heat measure, of a given motor. One disadvantage of this scheme is that use of an RC network to store an analog signal indicative of the motor's thermal characteristics requires different component values, such as the respective values for the capacitor and/or resistor which make up the RC network, for motors having different thermal characteristics. This follows since different time constants are required for modeling motors having different thermal characteristics. Thus, this scheme unfortunately results in a burdensome and costly proliferation of components and corresponding identifying catalog numbers for these components. Moreover, this proliferation of components and catalog numbers adds complexity and cost to field operations, such as maintenance and repairs. Another disadvantage in mimicking the thermal characteristics of the motor as a voltage being discharged from an RC network is that the transfer function of such network, in its simplest and most economical implementation, is limited to a plain exponential which may not be adequate for accurately modeling the relatively complex thermal characteristics of a given motor. Although some piecewise linear approximation can be achieved with more complex and costly discharge circuits, it will be apparent that none of the respective transfer functions of these analog circuits may accurately match, as suggested above, the relatively complex thermal characteristics of a given motor. Thus, it is desirable to provide a method and apparatus which advantageously and at relatively low cost overcomes the above-listed disadvantages.